The field of the invention pertains to fiber optic sensors instantaneously sensitive to pressure or stress in a manner that causes a beam of light to be reflectively modulated in response to changes in pressure or stress on the sensor.
In particular, in the automotive field, the ability to continuously monitor internal combustion engines for pressure fluctuations can significantly improve engine efficiency, performance, reliability and operating costs. Most importantly, the level of emissions can be reduced over the 100,000 mile effective life of the engine emissions control systems to be required by the United States Environmental Protection Agency. In addition, open and closed loop controls based on pressure information permit lean-burn engine operation, a wider tolerance to fuel octane and acceptance of alternative fuels.
Two combustion parameters, engine knock and misfire, have a particularly significant effect on overall engine performance. Combustion knock causes increased fuel consumption, reduced engine torque and engine deterioration if left uncorrected. Eventually severe damage such as perforated pistons can occur.
Misfire can result in catalyst damage and degradation that eventually cause vehicle exhaust emissions to no longer meet current or proposed emission standards. With a design 100,000 mile catalyst life, the failure to detect and correct misfire could result in operation of the vehicle for a lengthy period of time, possibly many years, with an ineffective catalyst.
The California Air Resources Board has recently proposed regulations which will require vehicles to be equipped with on-board emission monitoring systems. Such systems, in particular, will require misfire monitoring. The Environmental Protection Agency is also considering regulations to require such monitoring systems.
A low cost, reliable cylinder-selective combustion pressure sensor would permit knock and misfire detection separately for each cylinder. In addition to signalling the malfunction to the vehicle operator, a real time solid state engine control could adjust specific cylinder parameters to correct for the malfunction. The majority of the presently available or proposed knock and misfire detection techniques provide information that is not cylinder specific and therefore has limited utility for real time corrective controls. However, some recent patents disclose cylinder specific sensors. These sensors generally fall into two categories, luminosity detectors and pressure detectors.
U.S. Pat. No. 4,919,099 discloses a probe insertable into the engine cylinder combustion chamber. The probe includes a light conductive rod and fiber optic transmission bundle connected to an opto-electronic detector for instantaneous detection of the luminosity of the combustion gases within the combustion chamber. U.S. Pat. No. 5,052,214, in a similar manner, utilizes a fiber optic probe and transmission cable to sense and transmit the instantaneous luminosity to an opto-electronic detector. International Application Publication WO 89/11031 and European Application Publication EP-392-650-A also disclose optical luminosity probes for engine combustion chambers.
U.S. Pat. No. 4,781,059 discloses an optical fiber pressure sensor comprising a plurality of fibers to transmit light to the sensor tip and a second plurality of fibers to transmit light from the tip to an opto-electronic detector. The tip comprises a reflective diaphragm sensitive to pressure changes within the combustion chamber. U.S. Pat. No. 4,924,870 to applicant discloses an optical fiber pressure sensor tip comprising a single optical fiber. The single fiber carries dual light beams of differing wavelengths as input and the reflected return light beams. One wavelength serves as a reference signal that is reflected by an optical filter. The other wavelength passes through the optical filter and is reflected and modulated by a movable diaphragm sensitive to pressure changes. This particular fiber optic sensor is of very small size, being intended for the measurement of intra-vascular blood pressure in human patients.
Two other patents to the applicant, U.S. Pat. No. 4,932,262 and U.S. Pat. No. 4,932,263, disclose a well having an optical fiber passing therethrough. A pressure sensitive membrane encloses at least a portion of the well. The underside of the pressure sensitive membrane includes an optical grating that couples with the wavelength of the light beam in the optical fiber so as to modify the light beam in response to pressure induced movement of the membrane. By making the sensor with techniques common to the manufacture of integrated circuits on chips, the sensor may be made small and rugged enough to locate on a spark plug in direct exposure to the combustion chamber of an engine.
A published paper co-authored by the applicant is entitled "Microbending Losses of Metal Coated Single Mode, Multimode, and Cladding-Free Fibers," Society of Photo-Optical Instrumentation Engineers, Vol. 985 Fiber Optic and Laser Sensors VI (1988) and discloses the test results of microbending various optical fiber constructions. The test results indicate the various attenuations of light beams as a function of microbending displacement of the fibers and wavelength of the light beams.
A second published paper co-authored by the applicant is entitled "A Fiber Optic Sensor for Combustion Pressure Measurement in a Washer Configuration," Society of Photo-Optical Instrumentation Engineers, Vol. 840 Fiber Optic Systems for Mobile Platforms (1987), and discloses a washer configuration for placement between a spark plug and engine cylinder head. Changes in combustion chamber pressure cause changes in the preload on the washer configuration. The washer configuration comprises upper and lower serrated washer halves with a continuous loop of optical fiber placed between the serrated washer halves. One end of the loop extends to a source of light and the other end of the loop extends to a photodetector.